This invention relates generally to a signal input circuit, more specifically to an input circuit of an electronic measurement instrument such as an oscillosope or the like.
An input circuit of high precision quantitative measurement instrument like an oscilloscope is required to have a high impedance controllable attenuator stage for conditioning the wideband input signal without causing loading effect on the signal source to be measured.
For better understanding of the invention, a prior art oscilloscope is briefly described by reference to FIG. 1 which shows a simplified block diagram of a conventional oscilloscope. An input signal which varies in amplitude and frequency is applied to vertical input terminal 10 through a coaxial cable or an electrical probe. Coupled to input terminal 10 is input conditioning circuit 12 which may include a coupling circuit for selecting AC, DC or GND (ground), and a switchable attenuator. The output from signal conditioning circuit 12 is amplified by preamplifier 14, paraphase amplifier 16 for converting the single ended input signal into the push-pull or balanced output signal, and output amplifier 18 before being applied to the vertical deflection plates of cathode ray tube (CRT) 20. A fixed delay line (not shown) may be interposed between amplifiers 16, 18. Trigger generator 26 generates a sweep gate signal in synchronism with a trigger signal which may be the vertical input signal itself picked off from amplifier 16 in the internal (INT) trigger mode or an external trigger signal applied to external trigger input terminal 22 in the external (EXT) triggr mode. Either INT or EXT trigger mode is chosen by trigger mode switch 24. The sweep gate signal is applied to sweep generator 28 which generates a ramp signal for driving the horizontal deflection plates of CRT 20 after being amplified by horizontal amplifier 30. Although not shown in FIG. 1, an unblanking circuit is also employed for blanking or unblanking control of the electron beam.
In operation, the vertical input is first conditioned to appropriate amplitude by selecting attenuator in signal conditioning circuit 12 and then amplified to a certain large amplitude, e.g., 30 volts or so to provide the required vertical deflection of the electron beam. DC coupling is used to observe the wideband input signal up to its upper cut-off frequency, e.g. 100 MHz. In other words, both DC and AC signal components are observed in this mode. AC coupling is used to observe only AC components of the input signal, thereby eliminating DC and low frequency components below its lower cut-off frequency. GND is used to connect the input of the oscilloscope to ground while disconnecting the signal from the input signal source, thereby confirming the DC reference level on the CRT screen. The electron beam of the CRT is swept across the screen at a constant rate determined by the timing elements of ramp generator 28. The ramp signal is synchronized with the input signal to provide a stationary waveform of the input signal.
Impedance converter 14 is used to provide a high input impedance, e.g. 1M.OMEGA. for the minimum loading effect to the input signal source and also a low output impedance for ease of wideband signal amplification. For this end, impedance converter 14 may include a source follower input stage and an emitter follower output stage. Paraphase amplifier 16 is used to provide the balanced output suited for driving CRT deflection plates and also for picking off one part of the signal for triggering purpose.
One example of multi-stage attenuators for high frequency application is disclosed in U.S. Pat. No. 3,753,170, assigned to the assignee of the present invention. Each attenuator stage of the conventional attenuator is shown in FIG. 2. A pair of specially designed switches S.sub.1, S.sub.2 are connected in series between input terminal T.sub.1 and output terminal T.sub.2. Switches S.sub.1, S.sub.2 are disclosed in detail in U.S. Pat. No. 3,719,788, also assigned to the assignee of the present invention. Short bar SB is connected between upper terminals of switches S.sub.1 and S.sub.2 while the lower terminals thereof are connected via high impedance RC attenuator comprising resistors R.sub.1, R.sub.2 and and capacitors C.sub.1, C.sub.2, C.sub.3. Another variable capacitor C.sub.4 is coupled to output terminal T.sub.2 in shunt relation to the signal path. Also, connected to output terminal T.sub.2 is a parallel combination of resistor R.sub.0 and capacitor C.sub.0 representing respectively the input resistance and capacitance of impedance converter 14.
Switches S.sub.1 and S.sub.2 may be compared to single pole double-throw switches. In their upper position, input and output terminals T.sub.1 -T.sub.2 are directly coupled, thereby by-passing the attenuator. Capacitor C.sub.4 is used to adjusted the total input capacitance to a predetermined value (C.sub.in), thereby compensating for input impedance of impedance converter 14 which largely depends on the gate-to-drain capacitance of source follower transistor among some other stray capacitance. In the lower position of switches S.sub.1 and S.sub.2, the input signal is attenuated by the RC attenuator now interposed between input and output terminals T.sub.1 -T.sub.2. The attenuation ratio may be any desired value. In a case of attenuation factor of 5, R.sub.1 and R.sub.2 are chosen to 800 k.OMEGA. and 250 K.OMEGA., respectively. The input resistance remains unchanged (1M.OMEGA.) in this switch position. Shunt capacitor C.sub.1 is used to realize a wideband attenuator. The relation of R.sub.1 C.sub.1 =R.sub.2 //R.sub.0 (C.sub.0 +C.sub.4 +C.sub.2) must be maintained for the wideband attenuator. R.sub.2 //R.sub.0 is the parallel resistance of resistors R.sub.2 and R.sub.0. Variable resistor C.sub.2 is used for this purpose. Another variable capacitor C.sub.3 is used to equalize the input capacitance of the attenuator C.sub.in, thereby maintaining the frequency response of the attenuator circuit constant regardless of different attenuation factors.
FIG. 3 shows examples of conventional impedance converter 14 and paraphase amplifier 16 in FIG. 1. Impedance converter 14 consists of source follower input stage including field effect transistors (FETs) 36, 38 and resistor 40, and emitter follower output stage including bipolar transistor 42. The gate of FET 36 is coupled to input terminal 32 and returned to ground through resistor 34 which determines the input resistance. FET 38 and resistor 40 constitutes a current source. Paraphase amplifier 16 comprises two pairs of emitter coupled transistors 44-46 and 48-50. The emitter output of emitter follower transistor 42 is then supplied to the base of transistor 44 while a vertical position control signal from potentiometer 52 is supplied to the base of transistor 46. The bases of transistors 48 and 50 are connected to the emitters of transistors 44 and 46, respectively. The collectors of transistors 44-46 are coupled via terminals 54-56 to the input of vertical output amplifier 18, while the collector of transistor 50 provides a trigger signal from terminal 58.
As is understood from the foregoing description, conventional signal input circuit has a few disadvantages. Firstly, at least two specially designed switches are required for each attenuator stage, thereby making the switches and driving mechanism bulky, complicated, less reliable and also expensive because of the use of gold plated contacts. This is not negligible especially when using several cascaded attenuator stages as shown in the aforementioned prior art. Secondly, the vertical amplifier section is relatively complex to provide the required impedance conversion and to extract one part of the signal for triggering purposes. Thirdly, the frequency response is not flat over wide frequencies. In other words, fractions of the input signal components are lost due to capactive elements especially at high frequencies.
It is therefore the object of this invention to provide an improved signal input circuit free from the aforementioned disadvantages, and especially suited for a wideband test and measurement instrument like an oscilloscope.